Substances which are harmful to the environment are produced in combustion processes such as for example, fossil fuel power plants, internal combustion engines, gas turbines, and the like.
Internal combustion engines emit a large amount of unburned hydrocarbons and carbon monoxide during cold engine start-up. Release of hydrocarbons after starting an engine poses a special problem because at this point the exhaust gas temperature is not high enough for conversion to innocuous products to take place in the presence of conventional catalysts. Also, at start-up, the quantity of undesirable pollutants, especially hydrocarbons is at least an order of magnitude greater than after warm-up.
The catalysts utilized in catalytic converter systems are generally ineffective at ambient temperature and must reach high temperatures, often in the range of 300.degree. to 400.degree. C., before they are activated. Typically, the temperature of the catalyst is elevated by contacting it with the high temperature exhaust gases from the engine. Continuous contact with those gases and the exothermic nature of the oxidation reactions occurring at the catalyst, combine to raise the temperature of the catalyst and then maintain it at an elevated temperature.
Some methods of reducing cold start emissions which are practiced in the art are:
(1) locating the main catalyst or an auxiliary catalyst, referred to as a light-off catalyst close to the engine,
(2) use of electrically heated catalyst systems, and
(3) temporarily adsorbing hydrocarbons on zeolites and/or activated carbon wherein the zeolites are in the form of conventional pellets, beads or as washcoats on honeycombs.
There are disadvantages associated with each of these systems.
Placing the catalytic converter as close to the engine as physically possible is recommended to minimize the emission of pollutants during initial engine start-up because the closer the catalyst is to the engine, the hotter will be the exhaust gas when it contacts the catalyst and the more quickly the temperature of the catalyst will be raised to operating level. However, due to limitations of space in most vehicles, locating the total amount of catalyst in the system near the engine is difficult.
Heated catalyst systems, while effective in reducing hydrocarbons and carbon monoxide emissions, require a larger power supply and add additional weight. Furthermore, this would increase costs and cause unacceptable delays before the engine could be started with the assurance that undesirable pollution of the atmosphere would not occur.
Zeolite adsorption systems can reduce hydrocarbon emissions. Typically zeolites are crystallized as powders, formed into pellets, or zeolite powder may be embedded in or coated on porous ceramic pellets or beads, or the zeolite may be extruded into a porous structure, or embedded or coated on monolith ceramic substrates, or crystallized on the surface of a ceramic substrate. The present methods of utilizing zeolites have disadvantages. For example, in some methods such as coating, the amount of zeolite that can be coated on a substrate is limited by the surface area of the substrate. Under certain conditions, the coating is susceptible to attrition by abrasion. In order to increase the amount of zeolite, additional parts would have to be included in the catalyst system to hold relatively large quantities of zeolite, such as having the zeolite in a fixed bed through which the mixture to be converted passes. In the latter case, by-pass valving is needed, as discussed in U.S. Pat. No. 4,985,210. Bead/pellet reactors have large back pressures. Extruded monoliths of adsorbent material are not necessarily thermal shock resistant and are weaker than, for example, cordierite, which is the ceramic material usually used for coated substrates.
U.S. Pat. No. 4,985,210 relates to an exhaust gas purifying apparatus which employs zeolites for adsorbing harmful components in exhaust gas disposed at the upstream side of a catalytic converter so that when the exhaust gas temperature is not higher than a specific temperature, a harmful component is adsorbed by the adsorbent, and when the exhaust gas temperature exceeds the specific temperature, the harmful component desorbs from the adsorbent and is introduced into the catalytic converter. The system has additionally, activated carbon and a bypass in parallel, upstream of the adsorbent so that flow paths of exhaust gas are selectively switched from one to the other according to the temperature of the exhaust gas.
U.S. Pat. No. 5,078,979 relates to a process for treating an exhaust gas stream from an engine, especially during cold start. The process involves a molecular sieve bed over which the cold exhaust is flowed before flowing over a catalyst bed. Pollutants as hydrocarbons are adsorbed on the molecular sieve bed. When the molecular sieve bed reaches a temperature of about 150.degree. C., the pollutants are desorbed from the adsorbent bed and converted by the catalyst to innocuous compounds.
Oxides of nitrogen, commonly called NO.sub.x gases are troublesome type of pollutant because they produce acid rain.
Up to the present time, NO.sub.x emissions in automotive and stationary power plants have been controlled by reducing them to nitrogen by three way catalysts (TWC) such as [Pt and/or Pd with Rh]/CeO.sub.2 -Al.sub.2 O.sub.3 and selective catalytic reduction (SCR) with ammonia using a catalyst such as V.sub.2 O.sub.5 /TiO.sub.2 or [Fe,Cu, etc]-zeolite.
The catalyst can be utilized in various forms depending on the application and size and geometry of the system as discussed previously. In selective reduction applications, which are high dust applications, the coating and extruded SCR catalysts are susceptible to attrition by abrasion.
As the public's attention to the problem of air pollution grows, government emission standards are being made increasingly more restrictive. There remains a need for reducing the amounts of pollutants introduced into the atmosphere.
It would be an advancement in the art therefore, to have an apparatus and method for efficiently converting auto exhaust pollutants and stationary power plant NO.sub.x to innocuous products without any of the above described disadvantages.